1. Field of the Invention
The present invention relates to application of solder to circuitized substrates for receiving surface mountable electronic components. More specifically, it relates to solder application to fine pitch lands comprising footprints for directly receiving chips for attachment.
2. Background Art
As the size of printed circuitry decreases, the need for precise, repeatable techniques for processing such circuitry increases. Patterns of lands, or footprints, to which electronic devices are to be connected are comprised of ever shrinking sites as the pitch (distance between) ever narrowing component leads decreases. The desired result is to apply solder only to lands. Otherwise, bridging occurs. Further, current technology contemplates direct chip attachment (DCA) to circuitized substrates. In the case of flip-chips, DCA and other surface mount systems, footprint patterns are often in configurations other than perimetrical rectangles. Connections are necessary between outputs interior of the perimeter outline of a chip, as well as some surface mountable components.
The prior art includes much directed to the broad problem of applying solder--paste or web borne film --to circuit lands. Prior art application methods include screening paste through a mask with apertures corresponding to individual lands in a footprint.
It is also known, as shown in commonly assigned U.S. Pat. No. 4,832,255, to use a heated and reciprocally movable tool to controllably apply pressure and heat to a web borne film of solder positioned over conductive lands on a substrate. This reference also teaches providing sprocket holes in the web for facilitating advancement of the web for solder application at another site.